1. Field of Invention
The present invention relates to devices for heating and fixing toner particles on a recording medium, and more particularly, to a heating and fixing device, which heats and presses the recording medium uniformly.
2. Related Art
As for image forming apparatuses such as a photocopier, a printer, or a multi-function printer, toner particles are attached to a recording medium according to a predetermined output pattern, wherein the toner particles are transferred onto the recording medium by a photoconductor drum, and meanwhile, the toner particles are fixed on the recording medium by weak attracting force such as electrostatic force, and then they have to be pressed and heated by a heating and fixing device for being permanently fixed thereon.
U.S. Pat. No. 3,331,592 provides a heating and fixing device, which includes a heating roller and a pressing roller for heating and pressing the recording medium passing there-between. The heating roller is a heat-conductive hollow metal roller, and a heat source is disposed therein for heating the heating roller, such that the surface of the heating roller is heated up to an operating temperature that is sufficient for fixing the toner particles. The pressing roller is made of metal, and is coated with soft materials such as rubber or foam. Under heat and pressure generated by the heating roller, the toner particles are firmly fixed on the surface of the recording medium.
However, in such an approach, the heat source heats the heating roller by heat radiation, resulting in low efficiency of heat transfer and high energy consumption. And, it takes 30 to 40 seconds as a preheating time period to rise the temperature of the heating roller from the room temperature to the operating temperature. Moreover, it is not area contact but point contact between the heating roller and the recording medium, as a result, the recording medium curls up after being heated and pressed, thereby the probability of paper jam increases.
In view of the above problems of the heating roller, approaches of utilizing a heater resistor as a heat source are provided. For example, in U.S. Pat. No. 5,083,168, a heating resistor is used as the heat source, to reduce the preheating time period and enhance the efficiency of heat transfer. Furthermore, the heating resistor directly presses and heats the recording medium through a flexible film, and contacts with the recording medium by area contact, so as to eliminate from the problem that the paper curls up after being heated.
In U.S. Pat. No. 5,083,168, the heating resistor needs about 10 seconds to rise its temperature from the room temperature to the operating temperature. However, the heating resistor in U.S. Pat. No. 5,083,168 has a substrate made of ceramic, and a material of high resistance coefficient is coated on the substrate. It results in that the process for fabricating the heating resistor is quite complicated and thereby having a high cost. Moreover, it is relatively complicated to control the temperature of such heating resistor, thus, an additional temperature sensor is required to control the temperature to be raised, and thereby making the temperature control more complicated.
In U.S. Pat. No. 5,278,618, a heating resistor made of material of positive temperature coefficient is provided, which generates heat immediately after being powered on. After being powered on, the temperature of the heating resistor does not rise continuously, but achieving to a constant temperature. The temperature control method herein is much easier than that of U.S. Pat. No. 5,083,168, but the heating efficiency is not preferable.